As commonly known, a variety of aluminum alloy plate (hereinafter, aluminum may be referred to as “Al”) has heretofore been used generally as members of framework and components for transport machinery such as automobiles, ships, airplanes, and trains; and for industrial machinery, electrical equipment, buildings, structures, optical apparatus, and other machines and instruments according to characteristics particular to respective alloys.
These aluminum alloy plates are used for the abovementioned members of framework and components, in most cases, after press molding or other forming processing. In this respect, the Al—Mg series Al alloys which are excellent in the balance of strength and ductility are advantageous in point of high-level formability as may be required.
For the above reason, studies have been made concerning component composition and optimization of manufacturing conditions with respect to Al—Mg series Al alloy plates. As Al—Mg series Al alloys, those shown in JIS A5052, 5182, etc., represent typical composition of alloy components. But, even these Al—Mg series Al alloys are poorer in ductility and hence inferior in formability when compared with the cold-rolled sheet steel.
There is a way for the Al—Mg series Al alloys to enhance the balance of strength and ductility, if the Mg content is increased and the alloy is made up to such a high-Mg alloy as over 3%. However, such a high-Mg Al—Mg alloy is difficult to industrially manufacture by the normal manufacturing method where the ingot cast by the direct chill casting process or the like is taken through soaking and then hot rolling. The reason for the difficulty is that in the direct chill casting in which large strain occurs to the ingot, the ingot is susceptible to fracture because the solid and liquid phases coexistent temperature range is extensive, and deep wrinkles deriving from the thick oxide film take place on the molten metal. Also, in the normal hot rolling, the Al—Mg alloy suffers from significant decrease in ductility, becoming liable to fracture.
On the other hand, it is also difficult to perform hot rolling of a high-Mg Al—Mg series alloy at a low temperature avoiding a high temperature region where the abovementioned fracture may happen. The reason for the difficulty is that in such a low temperature rolling, deformation resistance of the material, that is, a high-Mg Al—Mg series alloy, increases remarkably to the extent that the product sizes available become extremely limited due also to the capability of the current rolling machine.
As an attempt to increase acceptable Mg amount in a high-Mg Al—Mg series alloy, it is also proposed to add Fe, Si, or any other third element. But, if the content of such third element is increased, rough and large intermetallic compounds are likely to be easily formed to the effect of lowering ductility of the aluminum alloy plate. Therefore, there was a limit in increasing acceptable Mg amount, and in fact, it was difficult to get Mg contained in an amount of 8% or over.
Therefore, the idea of manufacturing a high-Mg Al—Mg series alloy plate by a twin-roll type continuous casting method and other methods has hitherto been proposed in quite a variety. In the twin-roll type continuous casting method, molten Al-alloy metal is poured from a molten metal supply nozzle made of refractory into between a pair of rotating water-cooled casting molds (twin rolls). The molten metal is thus solidified, and immediately after solidification, the metal is rapidly cooled between the twin rolls giving birth to aluminum alloy sheets. This twin-roll type continuous casting method described above and the 3C method are among those well known.
The cooling rate of the twin-roll type continuous casting method is higher by 1-3 digits than the conventional DC casting method and the belt type continuous casting method. Because of this fast rate, the aluminum alloy sheets obtained have a very fine metallic structure and excellent workability such as press-formability. Also by the casting method, the aluminum alloy sheets are thus available in a relatively thin thickness as 1-13 mm. This means that, just as the conventional direct chill ingot (200 to 600 mm thickness), the processes of hot rough rolling, hot finish rolling, etc., can be dispensed with. Further, the homogenization treatment of ingot may sometimes be omissible.
Various propositions have heretofore been made with regard to examples specifying metallic structures with the intention to enhance formability of the high-Mg Al—Mg series alloy sheets manufactured by the twin roll continuous casting method. For example, an aluminum alloy sheet of Al—Mg series containing as high Mg content as 6-10% and having excellent features in mechanical properties with the intermetallic compounds the average diameter of which is 10 μm or less, is proposed (see Patent Document 1). Another proposition refers to an aluminum alloy sheet used for automobile body sheets having 300 pieces/mm2 or less of Al—Mg series intermetallic compounds of 10 μm or more, with average grain diameter ranging 10-70 μm. (see Patent Document 2).
With reference to 6000 series aluminum alloy, it was reported that casting of AA6016 aluminum alloy cast plates (1800W×1-2.5 mm thickness) was carried out by using the roll casting equipment called Speed Caster (see Non-patent Document 1).    [Patent Document 1] Japanese Patent Application Laid-open Publication No. 07-252571 (Scope of Claims pp. 1-2)    [Patent Document 2] Japanese Patent Application Laid-open Publication No. 08-165538 (Scope of Claims pp. 1-2)    [Non-patent Document 1] Continuous Casting, Proceedings of the International Conference on Continuous Casting of Non-Ferrous Metals, DGM2005, p 87